In the semiconductor integrated circuit (IC) fabrication, photolithography processes and etching processes are often processed repeatedly to form patterns on semiconductor substrates. An ordinary photolithography process can be carried out as following. First, a photoresist (PR) layer can be spin-coated on a silicon wafer. The silicon wafer coated with the photoresist layer can be selectively exposed to a certain light, such as an ultraviolet light, an X-ray, or an electron beam, etc. Going through a developing process, the photoresist layer that remains on the silicon wafer can form a photoresist pattern for protecting the covered area.
Since it is difficult to parse out a small-sized and hole-shaped pattern by one exposure using positive photoresists, negative photoresists are used in photolithography processes. A negative photoresist layer has a different optical characteristic compared to a positive photoresist layer. During exposure, a light curing reaction or a crosslinking reaction can occur in an exposed region of a negative photoresist layer. So during the developing process, the light-exposed region of the negative photoresist layer can be insoluble in development liquid.
However, as the critical dimensions of integrated circuits shrink, a photolithographic pattern formed by a negative photoresist layer may have defects in its shape and size.